Control of a modular converter with distributed energy storage devices

ABSTRACT

A device for converting an electrical current includes at least one phase module with an AC connection and at least one DC connection. A phase module branch is provided between each DC connection and each AC connection. Each phase module branch has a series connection made of sub-modules, which in turn include an energy accumulator each and at least one power semiconductor. Measuring sensors provide actual values and there are provided control means connected to the measuring sensors. The control can be easily adapted to any arbitrary number of sub-modules in each phase module branch. The control means include a current regulating unit and control units associated with a phase module branch each, wherein the current regulating unit is configured to provide branch target values for the control units. The control units are designed to produce control signals for the sub-modules.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an apparatus for conversion of anelectric current having at least one phase module which has an ACvoltage connection and at least one DC voltage connection with a phasemodule branch being formed between each DC voltage connection and the ACvoltage connection and with each phase module branch having a seriescircuit of submodules which each have an energy storage device and atleast one power semiconductor, having measurement sensors for provisionof actual values, and having closed-loop control means which areconnected to the measurement sensors and are designed to regulate theapparatus as a function of the actual values and predetermined nominalvalues.

The present invention likewise relates to a method for conversion of acurrent.

An apparatus such as this and a method such as this are already known,for example, from the article by A. Lesnicar and R. Marquardt “AnInnovative Modular Multilevel Converter Topology Suitable for a WidePower Range” which appeared at Powertech 2003. This discloses aconverter which is intended for connection to an AC voltage network. Theconverter has one phase module for each phase of the AC voltage networkto be connected to it, with each phase module having one AC voltageconnection and two DC voltage connections. Phase module branches extendbetween each DC voltage connection and the AC voltage connection, thusproviding a so-called 6-pulse bridge circuit. The module branchescomprise a series circuit of submodules which each comprise two powersemiconductors which can be turned off, each of which have back-to-backfreewheeling diodes connected in parallel with them. The powersemiconductors which can be turned off and the freewheeling diodes areconnected in series, with a capacitor being provided in parallel withsaid series circuit. Said components of the submodules are connected toone another such that the voltage at the two-pole output of eachsubmodule is either the capacitor voltage or zero.

The power semiconductors which can be turned off are controlled by meansof so-called pulse-width modulation. The closed-loop control means forcontrolling the power semiconductors have measurement sensors fordetection of currents, with current values being obtained. The currentvalues are supplied to a central control unit, which has an inputinterface and an output interface. A modulator, that is to say asoftware routine, is provided between the input interface and the outputinterface. Inter alia, the modulator has a selection unit and apulse-width generator. The pulse-width generator produces the controlsignals for the individual submodules. The power semiconductors whichcan be turned off are switched by the control signals, which areproduced by the pulse-width generator, from a switched-on state, inwhich current can flow via the power semiconductors which can be turnedoff, to a switched-off state, in which any current flow via the powersemiconductors which can be turned off is interrupted. In this case,each submodule has a submodule sensor for detection of a voltage acrossthe capacitor.

Further contributions to the control method for a so-called multilevelconverter topology are known from R. Marquardt, A. Lesnicar, J.Hildinger, “Modulares Stromrichterkonzept für Netzkupplungsanwendung beihohen Spannungen”, [Modular Converter Concept for Network CouplingApplication at high voltages], which appeared at the ETG Symposium inBad Nauenheim, Germany 2002, by A. Lesnicar, R. Marquardt, “A newmodular voltage source inverter topology”, EPE '03 Toulouse, France 2003and from R. Marquardt, A. Lesnicar “New Concept for High Voltage—ModularMultilevel Converter”, PESC 2004 Conference in Aachen, Germany.

A method for control of a polyphase converter with distributed energystorage devices has been disclosed in German Patent Application 10 2005045 090.3, which has currently not yet been published. The disclosedapparatus likewise has a multilevel converter topology with phasemodules which have two DC voltage connections, and one AC voltageconnection, which is arranged symmetrically at the center of each phasemodule. Each phase module is composed of two phase module branches,which extend between the AC voltage connection and one of the DC voltageconnections. Each phase module branch in turn comprises a series circuitof submodules, with each submodule comprising power semiconductors whichcan be turned off and freewheeling diodes connected back-to-back inparallel with them. Furthermore, each submodule has a unipolarcapacitor. The closed-loop control means are used to regulate the powersemiconductors and are also designed to adjust branch currents whichflow between the phase modules. By way of example, current oscillationscan be actively damped and operating points with relatively low outputfrequencies can be avoided by control of the branch currents.Furthermore, this makes it possible to uniformly load all thesemiconductor switches which can be turned off, and to balance highlyunbalanced voltages.

BRIEF SUMMARY OF THE INVENTION

One object of the invention is to provide an apparatus of the typementioned initially whose regulation can easily be matched to anydesired number of submodules in each phase module branch.

On the basis of the apparatus mentioned initially, the inventionachieves this object in that the closed-loop control means have acurrent regulation unit and drive units which are each associated withone phase module branch, with the current regulation unit being designedto provide branch nominal values for the drive units, and with the driveunits being connected between the submodules and the current regulationunit and being designed to produce control signals for said submodules.

On the basis of the method mentioned initially, the invention achievesthis object in that a current regulation unit is supplied with actualvalues and with nominal values, the current regulation unit definesbranch nominal values as a function of the actual values and the nominalvalues by means of a regulator, which branch nominal values arerespectively associated with one phase module branch, the branch nominalvalues are each supplied to a drive unit associated with said phasemodule branch, and each drive unit produces control signals for thesubmodules associated with it, as a function of the branch nominalvalues.

The apparatus according to the invention has closed-loop control meanswhich comprise a central current regulation unit. Said currentregulation unit is connected to the measurement sensors, which areprovided in order to detect electrical measurement variables, such ascurrent or voltage, with the measured values being supplied to theregulation system as so-called actual values. Furthermore, the currentregulation unit is supplied with nominal values to which the actualvalues are intended to be matched. If, for example, the nominal valuesare a predetermined nominal in-phase power, a change in the DC, forexample, in order to achieve the nominal in-phase power also results ina change to the alternating currents on the AC voltage side of theconverter. In other words, the actual values are coupled to one anotherto a high degree. The current regulation unit is therefore essentiallyused for decoupling of the regulator variables.

In contrast to the control methods known from the prior art, the controlsignals for the submodules, according to the invention, are not producedby a central current regulation unit. In fact, an independent drive unitis provided for each phase module branch. The current regulation unitproduces at least one branch nominal value for each drive unit. Thedrive unit then determines and produces the control signals for theindividual submodules on the basis of each branch nominal value. Theapparatus according to the invention therefore has closed-loop controlmeans which can easily be matched to a changing number of submodules.The current regulation unit is designed just for the number of phasemodule branches, which is independent of the number of submodules ineach phase module branch. Matching of the apparatus according to theinvention, that is to say of the converter according to the invention,to a different network voltage or DC voltage, with an increase in thenumber of submodules in consequence, therefore affects only the driveunits.

Each submodule advantageously has a submodule sensor which is connectedto the drive unit associated with that submodule and provides asubmodule actual value. The submodule actual value is supplied to thelinked drive unit, which then forms a submodule sum actual value byaddition of the submodule actual values which originate from submoduleswhich have been switched on, or in other words have been switched to beactive, by the drive unit. Only submodules which have been switched tobe active in a phase module branch make any contribution to thecorresponding submodule sum actual value of that phase module branch. Inthis case, the drive unit produces control signals for the individualsubmodules such that the submodule sum actual value corresponds asaccurately as possible to the branch nominal value provided by thecurrent regulation unit.

The submodule actual value is expediently an energy storage devicevoltage value Uc, which corresponds to a voltage across the energystorage device of the respective submodule. In this case, the branchnominal value is a branch voltage nominal value, that is to say anominal value for the total voltage across those submodules in a phasemodule branch which have been switched on or have been switched to beactive.

Each drive unit is advantageously connected to all the submodule sensorsof the phase module branch associated with it and is designed to providea branch energy actual value for the current regulation unit, with thebranch energy actual value being the sum of the submodule actual valuesof all the submodules, that is to say both the active submodules thatare switched on and the inactive submodules which have been turned offin the respective phase module branch. Furthermore, the branch energyactual value is itself used to determine the branch nominal value and inparticular the branch voltage nominal value.

According to one advantageous further development, the currentregulation unit is designed to provide a branch voltage nominal valueUp1ref, Up2ref, Up3ref, Un1ref, Un2ref, Un3ref for each drive unit.

The measurement sensors expediently comprise branch current sensorswhich are designed to measure phase module branch currents Izwg whichare flowing in the phase module branches. According to this advantageousfurther development, it is possible to regulate the phase module branchcurrents. The phase module branch currents Izwg comprise phase currentswhich are flowing on the AC voltage side of the phase module, that is tosay by way of example between a transformer for connection of an ACvoltage network to the apparatus according to the invention and the ACvoltage connection of the phase module. Furthermore, the phase modulebranch currents comprise DC components and circulating currents. If allthe phase module branch currents of the apparatus are known, thecirculating currents can be calculated. Since the circulating currentsare not externally visible, their regulation allows independentbalancing of the energy stored in the phase module branches. All degreesof freedom of the apparatus according to the invention can beeffectively used by regulation of the phase module branch currents andtherefore of the circulating currents. For example, active damping ofthe circulating currents considerably reduces the complexity which hasto be accepted in conjunction with passive elements, for example inconjunction with branch current inductors.

The current regulation unit is therefore advantageously designed toregulate the phase module branch currents Izwg.

According to one preferred further development, the nominal valuescomprise a reactive current nominal value Iqref, a in-phase currentnominal value Ipref and/or a DC nominal value Id. This allows a user tohandle the apparatus according to the invention in a particularly simplemanner. The user therefore just enters into the regulation system thein-phase power and reactive power to be transmitted. Reactive currentnominal values are determined from this with knowledge of the prevailingrated voltages.

With regard to the method according to the invention, it is advantageousfor the actual values to comprise branch energy actual values which aretransmitted by the drive units to the current regulation unit, with eachbranch energy actual value once again being the sum of the submoduleactual values of all the submodules in a phase module branch,irrespective of whether or not they have been switched on.

The branch nominal values are advantageously formed by a linearcombination of voltage intermediate nominal values. The voltageintermediate nominal values are very largely decoupled from one anotherand are used to set up a specific and clear regulation system.

According to one expedient further development relating to this, thevoltage intermediate nominal values comprise a DC voltage nominal valueUdc, with the DC voltage nominal value Udc being determined as afunction of the difference between a predetermined reference DC Idsolland a DC measured value Id obtained by measurement.

According to one expedient further development relating to this, a totalvoltage difference is determined by formation of the difference betweena predetermined sum voltage nominal value ucref and a total energymeasured value uc which is determined by addition of the voltages acrossall the energy storage devices of the converter, and the total voltagedifference is supplied to a regulator with a total energy discrepancycurrent value being obtained, with the total energy discrepancy currentvalue being added to a DC nominal value Idref, with the reference DCvalue Idsoll being obtained. The regulator which is used in this caseis, for example, a simple proportional regulator. However, otherregulators can also be used for the purposes of the invention. Accordingto this advantageous further development, the regulation system ensuresthat the energy which is stored in the energy storage device cannot beincreased beyond a predetermined level. This therefore prevents theapparatus according to the invention being destroyed by storage of anexcessively large amount of energy. It is obvious to a person skilled inthe art that, when adjusting the total energy of the apparatus accordingto the invention, that is to say of the converter, instead of additionof the measured voltage measured values of all the energy storagedevices of the apparatus, it is also possible to determine the energywhich is stored in the energy storage devices of the submodules, withenergy measured values being determined. uc would then correspond to thesum of the energy values of all the energy storage devices in theapparatus. By way of example, a measure of the energy value of an energystorage device is obtained from the voltage across the said energystorage device, simply by squaring said voltage.

The voltage intermediate nominal values for each drive unitadvantageously comprise network phase voltage nominal values Unetz1,Unetz2, Unetz3. The network phase voltage nominal values Unetz1, Unetz2,Unetz3 therefore essentially affect the apparatus such that a desiredphase current I1, I2, I3 is produced, which flows on the AC voltage sideof each phase module.

According to one expedient further development relating to this, thenetwork phase voltage nominal values Unetz1, Unetz2, Unetz3 aredetermined from phase current values which are obtained by measurementof the phase currents I1, I2, I3 on the AC voltage side of the phasemodules, as a function of current nominal values, by means of aregulator. According to this expedient further development, the phasecurrents are measured on the AC voltage side of the apparatus accordingto the invention. This can be done, for example, in the immediatevicinity of the AC voltage connections of the phase modules. For thispurpose, appropriate current transformers are made to interact with anAC conductor, with the AC conductors being connected to the AC voltageconnection. In contrast to this, however, the network current In1, In2and In3 can also be measured which is flowing in each phase of the ACnetwork which is connected to the AC voltage connections via the ACconductor and a transformer.

According to one expedient further development relating to this, thenetwork phase voltage nominal values Unetz1, Unetz2, Unetz3 aredetermined as a function of phase voltage measured values, which areobtained by measurement of the phase voltages U1, U2, U3 on the ACvoltage sides of the phase modules, as a function of nominal values, bymeans of a regulator. The network phase voltage nominal values Unetz1,Unetz2, Unetz3 can therefore also be obtained on the basis of themeasurement of the network voltages.

The voltage intermediate nominal values for each phase module branchadvantageously comprise a branch voltage intermediate nominal valueUzwgp1, Uzwgp2, Uzwgp3, Uzwgn1, Uzwgn2, Uzwgn3.

According to one expedient further development relating to this, thebranch voltage intermediate nominal values Uzwgp1, . . . , Uzwgn3 aredetermined as a function of extended branch current values Ip1, Ip2,Ip3, In1, In2, In3, by means of a regulator.

Each extended branch current value Ip1, Ip2, Ip3, In1, In2, In3 isadvantageously determined by formation of the sum of a phase modulebranch current measured value Izwgp1, . . . , Izwgn3, which is obtainedby detection of a phase module branch current flowing in the respectivephase module branch, of defined circulating-current nominal values Ikr1,Ikr2, Ikr3 and of defined balancing current nominal values Ibalp1, . . ., Ibaln3, with the balancing current nominal values Ibalp1, . . . Ibaln3being determined as a function of the branch energy actual values. Thenominal values which are predetermined in this regulation step, that isto say the circulating-current nominal values Ikr1, Ikr2, Ikr3, fordefinition of the circulating currents, which are components of thebranch currents, and the balancing current nominal values Ibalp1, . . ., Ibaln3 for definition of a balancing current, are added up togetherwith the phase module branch current measured value Izwg determined bymeasurement, with their sum value corresponding to said extended branchcurrent value Ip1, . . . , In3. The extended branch current value isthen expediently supplied to a regulator, which uses this to producebranch voltage intermediate nominal values Uzwg.

The branch voltage intermediate nominal values advantageously comprisean unbalanced nominal voltage Uasym.

According to one expedient further development relating to this, theunbalanced nominal voltage Uasym is defined by measurement of thevoltage between a positive DC connection and ground, with a positive DCvoltage value Udp being obtained, and by measurement of the voltagebetween a negative DC voltage connection and ground, with a negative DCvoltage value Udn being obtained, by formation of the difference betweenthe magnitudes of the positive and the negative DC voltage values, witha DC voltage difference Δud being obtained, and by application of the DCvoltage difference Δud to the input of a regulator, with the unbalancednominal voltage being obtained at the output of the regulator.

The branch voltage intermediate nominal values expediently havebalancing voltage nominal values Ubalp1, Ubalp2, Ubalp3, Ubaln1, Ubaln2,Ubaln3, with energy storage device voltage values Uc which correspond tothe voltages across the energy storage devices being detected, with theenergy storage device voltage values Uc of a phase module branch 6 p 1,6 p 2, 6 p 3, 6 n 1, 6 n 2, 6 n 3 being added, with branch energy actualvalues UcΣp1, UcΣp2, UcΣp3, UcΣn1, UcΣn2, UcΣn3 being obtained with thebranch energy actual values UcΣp1, UcΣp2, UcΣp3, UcΣn1, UcΣn2, UcΣn3being compared with one another and with a value derived from thecomparison being transmitted to a regulator, and with the balancingcompensation voltages Ubalp1, Ubalp2, Ubalp3, Ubaln1, Ubaln2, Ubaln3being tapped off at the output of the regulator. All the submodules of asubmodule branch are taken into account in the formation of the branchenergy actual values, irrespective of whether or not they are switchedon. The branch energy actual values thereby represent a measure of theenergy which is stored in a phase module branch. A person skilled in theart will be aware that, in this context, it is also possible to add upthe squares of the voltages across the energy storage devices, ratherthan the voltages themselves, and to form the branch energy actual valuein this way. Furthermore, it should also be noted that the apparatusaccording to the invention can also be balanced by the balancing currentnominal values Ibal described further above.

Extended branch current values Ip1, . . . , In3 are advantageouslybroken down as input variables of the regulation system into a networkcurrent component and a circulating current component in order todetermine the branch voltage intermediate nominal values Uzwgp1, . . . ,Uzwgn3. This breakdown allows the regulation steps which the currentregulating unit carries out to be set up clearly.

Each extended branch current value Ip1, . . . , In3 is advantageouslyregulated independently of the remaining branch current values Ip1, . .. , In3. This means that every extended branch current value issupplied, for example, together with expedient nominal values, in eachcase to a single regulator. The branch voltage intermediate nominalvalues Uzwgp1, . . . , Uzwgn3 can be tapped off at the output of theregulator.

According to a further advantageous refinement of the invention, phasecurrent values I1, I2, I3 are obtained by measurement of the phasecurrents on the AC voltage side and phase voltage values U1, U2, U3 areobtained by measurement of the phase voltages on the AC voltage sides ofthe phase module branches, auxiliary current values IHal, IHbe, aredetermined from the phase current values I1, I2, I3 and the phasevoltage values U1, U2, U3 as a function of nominal values, by means of aregulator, the auxiliary current values IHal, IHbe are added to orsubtracted from the extended branch current values Ip1, . . . , In3,with auxiliary sums or auxiliary differences being obtained, with theauxiliary sums and the auxiliary differences being applied to the inputof a regulator, and the branch voltage intermediate nominal valuesUzwgp1, . . . , Uzwgn3 being tapped off at the output of said regulator.In this case, by way of example, the regulator is a proportionalregulator.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

Further expedient refinements and advantages of the invention are thesubject matter of the following description of exemplary embodiments ofthe invention with reference to the figures of the drawing, in which thesame reference symbols refer to components having the same effect, andin which:

FIG. 1 shows a schematic illustration of one exemplary embodiment of anapparatus according to the invention,

FIG. 2 shows an equivalent circuit illustration of a submodule of anapparatus as shown in FIG. 1,

FIG. 3 shows the structure of the closed-loop control means of anapparatus as shown in FIG. 1,

FIG. 4 shows a linear combination of branch voltage intermediate nominalvalues in order to determine the branch voltage nominal values for thedrive units, illustrated schematically,

FIG. 5 shows a schematic illustration of the determination of networkphase voltage nominal values Unetz,

FIG. 6 shows a schematic illustration in order to illustrate the methodstep for determination of branch voltage intermediate nominal valuesUzwg from extended branch current values,

FIG. 7 shows a schematic illustration relating to the determination ofthe extended branch current values Ip as shown in FIG. 6,

FIG. 8 shows a schematic illustration of one possible way to producecirculating-current nominal values Ikr,

FIG. 9 shows a schematic illustration relating to the determination of aDC voltage nominal value Udc,

FIG. 10 shows an overview in order to illustrate the determination ofthe balancing voltage Uasym,

FIG. 11 shows a further possible way to produce branch voltageintermediate nominal values Uzwg,

FIG. 12 shows a further possible way to determine branch voltageintermediate nominal values Uzwg and

FIG. 13 shows a further possible way to determine branch voltageintermediate nominal values Uzwg.

DESCRIPTION OF THE INVENTION

FIG. 1 shows one exemplary embodiment of the apparatus 1 according tothe invention, which comprises three phase modules 2 a, 2 b and 2 c.Each phase module 2 a, 2 b and 2 c is connected to a positive DC voltageline p and to a negative DC voltage line n, in such a way that eachphase module 2 a, 2 b, 2 c has two DC voltage connections. Furthermore,a respective AC voltage connection 3 ₁, 3 ₂ and 3 ₃ is provided for eachphase module 2 a, 2 b and 2 c. The AC voltage connections 3 ₁, 3 ₂ and 3₃ are connected via a transformer 4 to a three-phase AC voltage network5. The phase voltages between the phases of the AC voltage network 5 areU1, U2 and U3, with network currents In1, In2 and In3 flowing. The phasecurrent on the AC voltage side of each phase module is denoted I1, I2and I3. The DC is I_(d). Phase module branches 6 p 1, 6 p 2 and 6 p 3extend between each of the AC voltage connections 3 ₁, 3 ₂ or 3 ₃ andthe positive DC voltage line p. The phase module branches 6 n 1, 6 n 2and 6 n 3 are formed between each AC voltage connection 3 ₁, 3 ₂, 3 ₃and the negative DC voltage line n. Each phase module branch 6 p 1, 6 p2, 6 p 3, 6 n 1, 6 n 2 and 6 n 3 comprises a series circuit ofsubmodules, which are not illustrated in detail in FIG. 1, and aninductance, which is denoted L_(Kr) in FIG. 1.

In FIG. 2, the series circuit of the submodules 7 and, in particular,the design of the submodules are illustrated in more detail in the formof an electrical equivalent circuit, although only the phase modulebranch 6 p 1 has been picked out in FIG. 2. The remaining phase modulebranches are, however, of identical design. As can be seen, eachsubmodule 7 has two power semiconductors T1 and T2, which are connectedin series and can be turned off. By way of example, power semiconductorswhich can be turned off are so-called IGBTs, GTOs, IGCTs or the like.These are known per se to a person skilled in the art and therefore donot need to be descried in detail at this point. Each powersemiconductor T1, T2 which can be turned off has a freewheeling diodeD1, D2 connected back-to-back in parallel with it. A capacitor 8 isconnected as an energy storage device in parallel with the seriescircuit of the power semiconductors T1, T2 which can be turned off andthe freewheeling diodes D1 and D2. Each capacitor 8 is charged on aunipolar basis. Two voltage states can now be produced at the two-poleconnecting terminals X1 and X2 of each submodule 7. For example, if thedrive unit 9 is producing a drive signal, by means of which the powersemiconductor T2 which can be switched off is switched to itsswitched-on state, in which a current can flow via the powersemiconductor T2, the voltage between the terminals X1, X2 of thesubmodule 7 is zero. In this case, the power semiconductor T1 which canbe turned off is in its switched-off state, in which any current flowvia the power semiconductor T1 which can be turned off is interrupted.This prevents the discharging of the capacitor 8.

If, in contrast, the power semiconductor T1 which can be turned off isin its switched-on state, but the power semiconductor T2 which can beturned off has been switched to its switched-off state, the entirecapacitor voltage Uc is applied between the terminals X1, X2 of thesubmodule 7.

The exemplary embodiment of the apparatus according to the invention asshown in FIGS. 1 and 2 is also referred to as a so-called multilevelconverter. A multilevel converter such as this is, for example, suitablefor driving electrical machines, such as motors or the like.Furthermore, a multilevel converter such as this is also suitable foruse in the power distribution and transmission field. By way of example,the apparatus according to the invention is used as a back-to-back linkwhich comprises two converters connected to one another on the DCvoltage side, with each of the converters being connected to an ACvoltage network. Back-to-back links such as these are used to exchangeenergy between two power distribution networks when, for example, thepower distribution networks have a different frequency, phase angle,star-point connection or the like. Furthermore, applications may beconsidered in the field of power factor correction, as so-called FACTS(Flexible AC Transmission Systems). High-voltage DC transmission overlong distances is also feasible using multilevel converters such asthese. Because of the range of different application options, there area large number of different operating voltages to which the respectiveapparatus according to the invention has to be matched. For this reason,the number of submodules may vary from a few up to several hundredsubmodules 7. In order to allow this very large number of submodules 7to access closed-loop control means which can easily be matched to thedifferent numbers of submodules 7, the invention has a structure whichis different to that according to the prior art.

FIG. 3 illustrates said structure of the closed-loop control means. Theclosed-loop control means comprise a current regulation unit 10 as wellas drive units 9 p 1, 9 p 2, 9 p 3 and 9 n 1 and 9 n 2 and 9 n 3. Eachof the drive units is associated with a respective phase module branch 6p 1, 6 p 2, 6 p 3, 6 n 1, 6 n 2 and 6 n 3. For example, the drive unit 9p 1 is connected to each submodule 7 of the phase module branch 6 p 1and produces the control signals for the power semiconductors T1, T2which can be turned off. A submodule voltage sensor, which is notillustrated in the figures, is provided in each submodule 7. Thesubmodule voltage sensor is used to detect the capacitive voltage acrossthe capacitor 8 of the submodule 7, with a capacitor voltage value Ucbeing obtained as the submodule actual value. The capacitor voltagevalue Uc is made available to the respective drive unit, in this case 9p 1. The drive unit 9 p 1 therefore receives the capacitor voltagevalues of all the submodules 7 of the phase module branch 6 p 1associated with it, and adds these to obtain a branch energy actualvalue UcΣp1, which is likewise associated with the phase module branch 6p 1. Furthermore, each evaluation unit 9 p determines a submodule actualvalue. The submodule actual value is calculated from the sum of theactive submodules between whose output terminals X1 and X2 thecapacitive voltage Uc is produced. Inactive submodules 7 between whoseoutput terminals X1, X2 the voltage is zero are ignored in the formationof the submodule actual value. In particular, the branch energy actualvalue UcΣp1 is supplied to the current regulation unit 10.

In addition, the current regulation unit 10 is connected to variousmeasurement sensors, which are not illustrated in the figures. Forexample, current transformers which are arranged on the AC voltage sideof the phase modules 2 a, 2 b, 2 c are used to produce and supply phasecurrent measured values I1, I2, I3, and current transformers which arearranged on each phase module are used to produce and supply phasemodule branch current measured values Izwg, and a current transformerwhich is arranged in the DC circuit of the converter is used to provideDC measured values Id. Voltage converters in the AC network providephase voltage measured values of the phase voltages U1, U2, U3 and DCvoltage converters provide positive DC voltage measured values of thepositive DC voltage Udp and negative DC voltage measured values of thenegative DC voltage Udn, with the positive DC voltage measured valuesUdp corresponding to a DC voltage between the positive DC voltageconnection p and ground, and with the negative DC voltage measuredvalues Udn corresponding to a voltage between the negative DC voltageconnection and ground. The negative DC voltage is negative. The positiveDC voltage is positive.

Furthermore, nominal values are supplied to the current regulation unit10. In the exemplary embodiment shown in FIG. 3, an in-phase currentnominal value Ipref and a reactive current nominal value Iqref aresupplied to the regulation unit 10. Furthermore, a DC voltage nominalvalue Udref is applied to the input of the current regulation unit 10.Instead of the DC voltage nominal value Udref, a DC nominal value Idrefcan also be used for further regulation purposes. These two nominalvalues can therefore be interchanged with one another.

The nominal values Ipref, Iqref and Udref and said measured valuesinteract with one another with the use of different regulators with abranch voltage nominal value Up1ref, Up2ref, Up3ref, Un1ref, Un2ref,Up3ref being produced for each drive unit 9 p 1, 9 p 2, 9 p 3, 9 n 1, 9n 2 and 9 n 3, respectively. Each drive unit 9 produces control signalsfor the submodules 7 associated with it, as a result of which thevoltage Up1, Up2, Up3, Un1, Un2, Un3 across the series circuit of thesubmodules corresponds as closely as possible to the respective branchvoltage nominal value Up1ref, Up2ref, Up3ref, Un1ref, Un2ref, Un3ref.The voltage Up, Up2, Up3, Un1, Un2, Un3 is referred to as the submodulesum actual value.

The other figures illustrate how the current regulation unit 10 formssuitable branch voltage nominal values Up1ref, Up2ref, Up3ref, Up1ref,Un2ref, Un3ref from its input values. For example, FIG. 4 shows that thebranch voltage nominal value Up1ref is calculated by linear combinationof a network phase voltage nominal value Unetz1, a branch voltageintermediate nominal value Uzwgp1, a DC voltage nominal value Udc, abalancing voltage nominal value Uasym and a balancing voltage nominalvalue Udalp1. This is done for each of the phase module branches 6 p 1,6 p 2, 6 p 3, 6 n 1, 6 n 2 and 6 n 3 independently of one another.

FIG. 5 shows how the network phase voltage nominal values Unetz1, Unetz2and Unetz3 are determined from the phase current measured values I1, I2and I3 and from the phase voltage measured values U1, U2, U3. Since thephase current measured values result in a total of zero, the phasecurrent measured values I1, I2, I3 of the three phases can be projectedinto a two-phase vector system α, β. This is done by means of theconversion unit 11. A corresponding situation applies to the phasevoltage measured values U1, U2, U3. The measured values are thensupplied to a regulator 12 which produces the network phase voltagenominal values Unetz1, Unetz2, Unetz3 as a function of the in-phasecurrent nominal value Ipref and as a function of a reactive currentnominal value Iqref, once again with a conversion unit 11 being used toconvert the two-dimensional network phase voltage nominal values tothree-dimensional network phase voltage nominal values.

FIG. 6 shows how the branch voltage intermediate nominal values Uzwgp1,Uzwgp2 and Uzwgp3, as well as Uzwgn1, Uzwgn2 and Uzwgp3, which areannotated for the first time in figure 4, are formed. This is done onthe basis of extended branch current values Ip1, Ip2, Ip3, In1, In2,In3, whose determination is described in the following text. The sixextended branch current values Ip1, Ip2, Ip3, In1, In2, In3 are onceagain converted, as described above, by a conversion unit 11 to fourextended two-dimensional branch current values α, β. A regulator 12,which in this case is a simple proportional regulator, then in each caseensures, together with the conversion unit 11, conversion to so-calledbranch voltage intermediate nominal values Uzwgp1, Uzwgp2 and Uzwgp3, aswell as Uzwgn1, Uzwgp2 and Uzwgp3.

The determination of the extended branch current values Ip1, Ip2, Ip3,In1, In2, In3 is illustrated in FIG. 7. The extended branch currentvalues Ip1, Ip2, Ip3, In1, In2, In3 with respect to the phase modulebranch 6 p 1 are nothing more than the sum of phase module branchcurrent measured values Izwgp1, a circulating-current nominal value Ikr1and a balancing current nominal value Ibal1, formed by a currenttransformer. The circulating-current nominal values Ikr1, Ikr2 and Ikr3can be dynamically preset via a control station, which is notillustrated in the figures. A corresponding situation applies to thebalancing current nominal values Ibalp1, Ibalp2 and Ibalp3. Eachextended branch current value Ip1 therefore comprises both measuredvalues and nominal values. The energy which is stored in each of thephase module branches is distributed in a balanced manner by means ofthe balancing nominal values.

FIG. 8 shows one advantageous example relating to the production ofsuitable circulating-current nominal values Ikr1, Ikr2, Ikr3. First ofall, the angular frequency of the network voltage ω is multiplied by afactor of 2. The cosine or the negative sine of the argument 2ω is thenformed, and is then multiplied by an amplitude Amp. A respectivecirculating-current nominal value Ikr1, Ikr2, Ikr3 is then determinedfrom the two variables, using a conversion unit 11, for each of thethree phase modules.

FIG. 9 illustrates the determination of the DC voltage nominal valueUdc. Udc is determined on the basis of a measured DC value Id and areference DC value Idsoll, with the process of determining the referenceDC value Idsoll being described in the following text. First of all, thedifference is formed between the measured DC value Id and the referenceDC value Idsoll. The difference is then supplied to a proportionalregulator or to a proportional/integral regulator, that is to say a PIregulator, 12, at whose output the DC voltage nominal value Udc can betapped off.

The lower part of FIG. 9 shows how the reference DC value Idsoll can bedetermined. This is done by first of all forming a total energy measuredvalue uc which is equal to the sum of all the capacitor voltage valuesUc of the apparatus 1 according to the invention. The total energymeasured value Uc therefore represents a measure of the energy stored inthe respective converter. A measure such as this can be derived in anyother desired manner. In order to ensure that said energy does notbecome excessively high, the total energy measured value uc is comparedwith a sum voltage nominal value ucref by using a subtractor 13 to formthe difference. Said difference is then supplied to a regulator 12, atwhose output a total energy discrepancy current value can be read, whichis supplied to an adder 14. The adder 14 forms the sum of the totalenergy discrepancy current value and a DC nominal value Idref, which isknown by the current regulation unit, with the reference DC nominalvalue Idsoll being obtained. This determination of the DC voltagenominal value Udc therefore makes it possible for the regulation systemto avoid the storage of an excessive amount of energy in the capacitors8 in the converter 1.

FIG. 10 physically illustrates the significance of an unbalanced voltageUasym. FIG. 10 illustrates a star-point former 15 by means of dashedlines on the AC voltage side of the phase modules of the apparatus 1according to the invention. A voltage divider 16 can likewise be seen inthe form of dashed lines in the DC voltage circuit p, n, having the sameresistance on both sides of the potential point N_(GS). The unbalancedvoltage Uasym is the voltage between the star point N_(TR) of thestar-point former 15 and the potential point N_(GS). This is determinedfirst of all by measurement of the voltage between the positive DCvoltage p and ground, with a positive DC voltage value Udp beingobtained, and by measurement of the voltage between the negative pole ofthe DC voltage and ground, with a negative DC voltage value Udn beingobtained. The difference between the magnitudes of the negative DCvoltage value Udn and the positive DC voltage value Udp is then formed,thus resulting in a DC voltage difference ΔUd. The DC voltage differenceΔUd is applied to the input of a regulator, with a DC voltage nominaldifference also being preset for the regulator, thus resulting in avalue being produced at the output of the regulator, by means of whichthe regulation process minimizes the difference between the DC voltagedifference and the DC voltage nominal difference. The balancing nominalvoltage Uasym can be tapped off at the output of the regulator and canbe applied to other voltage intermediate nominal values, on the basis ofthe linear combination as illustrated in FIG. 4.

The balancing voltage nominal values Ubalp1, Ubalp2, Ubalp3, Ubaln1,Ubalp2 and Ubalp3 are determined as follows: first of all, the capacitorvoltage values Uc are determined by measurement of the voltage acrossthe capacitors in the submodules 7, and are added, with branch energyactual values UcΣp1, UcΣp2, UcΣp3, UcΣn1, UcΣn2, UcΣn3 being obtained.All the submodules of the respective phase module branch are taken intoaccount in this process, to be precise irrespective of whether therespective submodule is or is not switched on. The branch energy actualvalue is therefore a measure of the energy stored in the phase module.The branch energy actual values are therefore respectively associatedwith a phase module branch 9 p 1, 9 p 2, 9 p 3, 9 n 1, 9 n 2 and 9 n 3.The branch energy actual values UcΣp1, UcΣp2, UcΣp3, UcΣn1, UcΣn2, UcΣn3are compared with one another, and a value is derived from thecomparison. This value is then transmitted with a nominal value to aregulator, at whose output the balancing voltage nominal values Ubalp1,Ubalp2, Ubalp3, Ubaln1, Ubaln2, Ubaln3 can be tapped off.

FIG. 11 shows a further possible way to determine the branch voltageintermediate nominal values Uzwgp1, Uzwgp2 and Uzwgp3 on the basis ofthe extended branch current values Ip1, Ip2, Ip3 and In1, In2 and In3.First of all, the extended branch current values Ip1, Ip2, Ip3 and In1,In2 and In3 are converted by conversion units 11 from athree-dimensional vector space to a two-dimensional vector space α, β.The regulation process for a network current component and for acirculating current component are then carried out independently of oneanother. Network current components iNal and the network currentcomponent iNbe are thus formed by a suitable linear combination and aresupplied to a regulator 12 with two nominal values, which are notillustrated. The regulator 12 forms an α-value uNal and a β-value uNbefor the network current component at its output. The regulation processis carried out in a corresponding manner for the circulating currentcomponent. This results in circulating current values ikral and ikrbe,in which case network voltage circulating components ukral and ukrbe canbe tapped off at the output of the regulator 12, which is shown at thebottom in FIG. 11. The branch voltage intermediate nominal values Uzwgp1to Uzwgp3 are obtained by suitable linear combination and conversion tothree-phase space.

In contrast to the method proposed in FIG. 11, the branch voltageintermediate nominal values Uzwgp1 to Uzwgn3 can also be determinedindependently of one another on the basis of the extended current valuesIp1, Ip2 and Ip3. To this end—as can be seen in FIG. 12—a separateregulator 12 is provided for each extended branch current value Ip1 toIn3, with the extended branch current values Ip1 to In3 being determinedas stated above. By way of example, the regulator 12 is a proportionalregulator.

FIG. 13 shows a further method for determination of the branch voltageintermediate nominal values Uzwgp1, . . . , Uzwgn3. First of all, thephase current measured values I1, I2 and I3 and phase voltage measuredvalues U1, U2 and U3 are converted from three-phase space to a two-phasespace α, β, and the respective converted measured values are supplied toa vector regulator 12. An in-phase current nominal value Ipref and thereactive current nominal value Iqref are also supplied to the regulator12. At its output, the vector regulator 12 produces auxiliary currentvalues IHal and IHbe on the basis of the difference between the in-phasecurrent nominal value and the in-phase current measured value, asdetermined from the measured values, and, at the same time, thedifference between the reactive current measured values and the reactivecurrent nominal value being minimal. The auxiliary current values IHal,IHbe are then linearly combined, as indicated in FIG. 13, with extendedbranch current values Ip1, . . . , Ip3. As has already been stated, theextended branch current values Ip1 to In3 comprise nominal currentvalues by means of which the regulator 12 produces two-dimensionalbranch voltage values α, β, and the conversion unit 11, finally,produces three-phase branch voltage intermediate nominal values Uzwgp1to Uzwgp3.

1. An apparatus for converting an electric current, comprising: at leastone phase module having an AC voltage connection and at least one DCvoltage connection with a phase module branch formed between each saidDC voltage connection and each said AC voltage connection, each phasemodule branch including a series circuit of submodules each having anenergy storage device and at least one power semiconductor; measurementsensors for providing actual values; and closed-loop control meansconnected to said measurement sensors and configured to regulate theapparatus in dependence on the actual values and predetermined nominalvalues, said control means having a current regulation unit and driveunits each associated with a respective said phase module branch;wherein said current regulation unit is configured to provide branchnominal values for said drive units, and wherein said drive units areconnected between said submodules and said drive unit is configured toproduce control signals for said submodules.
 2. The apparatus accordingto claim 1, wherein each said submodule has a submodule sensor connectedto said drive unit associated with the respective said submodule andoutputting a submodule actual value.
 3. The apparatus according to claim2, wherein the submodule actual value is an energy storage devicevoltage value Uc, defined by a voltage across said energy storage deviceof the respective said submodule.
 4. The apparatus according to claim 1,wherein each drive unit is connected to all said submodule sensors ofsaid phase module branch associated therewith and is configured toprovide a sum actual value for said current regulation unit, with a sumactual value being a sum of all the submodule actual values of therespective said phase module branch.
 5. The apparatus according to claim1, wherein said current regulation unit is configured to provide abranch voltage nominal value Up1ref for each said drive unit.
 6. Theapparatus according to claim 1,wherein said measurement sensors includebranch current sensors configured to measure phase branch currents lzwgflowing in said phase module branches.
 7. The apparatus according toclaim 6, wherein said current regulation unit is configured to regulatethe phase branch currents lzwg.
 8. The apparatus according to claim 1,wherein the nominal values are selected from the group consisting of areactive current nominal value Iqref and an in-phase current nominalvalue Ipref, and/or a DC nominal value ld.
 9. A method for converting acurrent, which comprises: providing a converter with at least one phasemodule having at least one DC voltage connection and an AC voltageconnection, with a phase module branch formed between each DC voltageconnection and AC voltage connection and having a series circuit ofsubmodules each having an energy storage device and at least one powersemiconductor; supplying a current regulation unit with actual valuesand with nominal values, the current regulation unit defining branchnominal values in dependence on the actual values and the nominal valuesby way of a closed-loop controller, wherein the branch nominal valuesare respectively associated with one phase module branch; and supplyingeach of the branch nominal values to a drive unit, and producing witheach drive unit control signals for the submodules associated therewith,in dependence on the module branch nominal values.
 10. The methodaccording to claim 9, wherein the actual values comprise branch energyactual values which are transmitted by the drive units to the currentregulation unit, and the method comprises forming the branch energyactual values by adding submodule actual values detected in thesubmodules.
 11. The method according to claim 9, which comprises formingthe branch nominal values by a linear combination of voltageintermediate nominal values.
 12. The method according to claim 11,wherein the voltage intermediate nominal values comprise a DC voltagenominal value, and the method comprises determining the DC voltagenominal value as a function of a difference between a predeterminedreference DC value ldsoll and a DC measured value Id obtained bymeasurement.
 13. The method according to claim 12, which comprisesdetermining a total voltage difference by formation of a differencebetween a predetermined sum voltage nominal value ucref and a totalenergy measured value uc determined by addition of the voltages acrossall the energy storage devices of the converter, and supplying the totalvoltage difference to a regulator with a total energy discrepancycurrent value being obtained, and adding the total energy discrepancycurrent value to a DC nominal value ldref, to obtain the reference DCvalue ldsoll being obtained.
 14. The method according to claim 11,wherein the voltage intermediate nominal values for each drive unitcomprise a network phase voltage nominal value Unetz1, Unetz2, Unetz3.15. The method according to claim 14, which comprises determining thenetwork phase voltage nominal values Unetz1, Unetz2, Unetz3 from phasecurrent values l1, l2, l3, which are obtained by measuring the phasecurrents on the AC voltage sides of the phase modules as a function ofcurrent nominal values, by means of a regulator.
 16. The methodaccording to claim 14, which comprises determining the network phasevoltage nominal values Unetz1, Unetz2, Unetz3 as a function of phasevoltage values U1, U2, U3, which are obtained by measuring the phasevoltages on the AC voltage sides of the phase modules as a function ofnominal values, by means of a regulator.
 17. The method according toclaim 11, wherein the voltage intermediate nominal values for each phasemodule branch comprise a branch voltage intermediate nominal valueUzwgp1, Uzwgp2, Uzwgp3, Uzwgn1, Uzwgn2, Uzwgn3.
 18. The method accordingto claim 11, which comprises determining the branch voltage intermediatenominal values Uzwgp1, Uzwgp2, Uzwgp3, Uzwgn1, Uzwgn2, Uzwgn3 as afunction of extended branch current values lp1, lp2, lp3, ln1, ln2, ln3,by means of a regulator.
 19. The method according to claim 17, whichcomprises breaking down extended branch current values lp1, lp2, lp3,ln1, ln2, ln3 as input variables of the regulation system into a networkcurrent component and a circulating current component, in order todetermine the branch voltage intermediate nominal values Uzwgp1, Uzwgp2,Uzwgp3, Uzwgn1, Uzwgn2, Uzwgn3.
 20. The method according to claim 18,which comprises determining each extended branch current valuelp1,lp2,lp3,ln1, ln2, ln3 by forming a sum of a phase module branchcurrent measured value lzwgp1, lzwgp2, lzwgp3, lzwgn1, lzwgn2, lzwgn3,which is obtained by detecting a phase module branch current flowing inthe respective phase module branch, of defined circulating-currentnominal values lkr1, lkr2, lkr3 and of defined balancing current nominalvalues lbalp1, lbalp2, lbalp3, lbaln1, lbaln2, lbaln3, with thebalancing current nominal values lbalp1, lbalp2, lbalp3, lbaln1, lbaln2,lbaln3 being determined as a function of the branch energy actual valuesof the phase module branches.
 21. The method according to claim 18,which comprises regulating each extended branch current value lp1, lp2,lp3, ln1, ln2, ln3 independently of the remaining branch current valueslp1, lp2, lp3, ln1, ln2, ln3.
 22. The method according to claim 18,which comprises: obtaining phase current values l1, l2, l3 by measuringthe phase currents on the AC voltage side and obtaining phase voltagevalues U1, U2, U3 by measuring the phase voltages on the AC voltagesides of the phase module branches; determining auxiliary current valueslHal, lHbe from the phase current values l1, l2, l3 and the phasevoltage values U1, U2, U3 as a function of nominal values, by means of aregulator; adding the auxiliary current values lHal, lHbe to orsubtracting the auxiliary current values lHal, lHbe from the extendedbranch current values lp1, lp2, lp3, ln1,ln2, ln3, to obtain auxiliarysums or auxiliary differences, respectively; and applying the auxiliarysums and/or auxiliary differences to an input of the regulator, andtapping off the branch voltage intermediate nominal values Uzwgp1,Uzwgp2, Uzwgp3, Uzwgn1, Uzwgn2, Uzwgn3 at an output of the regulator.23. The method according to claim 11, wherein the branch voltageintermediate nominal values have an unbalanced nominal voltage Uasym.24. The method according to claim 23, which comprises defining theunbalanced nominal voltage Uasym by: measuring a voltage between apositive DC connection and ground, to obtain a positive DC voltage valueUdp, and measuring a voltage between a negative DC voltage connectionand ground, to obtain a negative DC voltage value Udn; forming adifference between the magnitude values of the positive DC voltage valueUdp and the negative DC voltage value Udn, to obtain a DC voltagedifference Δud; and applying the DC voltage difference Δud to an inputof a regulator, to obtain the unbalanced nominal voltage Uasym at anoutput of the regulator.
 25. The method according to claim 11, whereinthe branch voltage intermediate nominal values include balancing voltagenominal values Ubalp1, Ubalp2, Ubalp3, Ubaln1, Ubaln2, Ubaln3, and themethod which comprises: detecting energy storage device voltage valuesUc that correspond to the voltages across the energy storage devices;summing the energy storage device voltage values Uc of a phase modulebranch to obtain branch energy actual values UcΣp1, UcΣp2, UcΣp3, UcΣn1,UcΣn2, UcΣn3, and comparing the branch energy actual values UcΣp1,UcΣp2, UcΣp3, UcΣn1, UcΣn2, UcΣn3 with one another; transmitting a valuederived from the comparison to a regulator, and tapping off thebalancing compensation voltages Ubalp1, Ubalp2, Ubalp3, Ubaln1, Ubaln2,Ubaln3 at the output of the regulator.